A heat-absorbing glass plate is required to have a low solar transmittance and a high visible light transmittance. That is, it is required to have a high ratio Tv/Te of the visible light transmittance (by illuminant A, 2° visual field) as defined in JIS R3106 (1998) (hereinafter sometimes referred to as Tv) calculated as 4 mm thickness of the glass plate to the solar transmittance as defined in JIS R3106 (1998) (hereinafter sometimes referred to as Te) calculated as 4 mm thickness of the glass plate.
Further, the heat-absorbing glass plate is required to provide a substantially achromatic (gray) transmitted light, that is, to have a low excitation purity Pe as defined in JIS Z8701 (1999) (hereinafter sometimes referred to as Pe), in order that an object or a scene can be seen in the actual colors when observed through the glass plate.
As a heat-absorbing glass plate having a low Pe, for example, the following glass plate has been proposed.
(1) A heat-absorbing glass plate (Patent Document 1) containing, as represented by mass % based on the following components:                SiO2: 66 to 75%,        Na2O: 10 to 20%,        CaO: 5 to 15%,        MgO: 0 to 5%,        Al2O3: 0 to 5%,        K2O: 0 to 5%,        total iron as calculated as Fe2O3: 0.30 to 0.70%,        divalent iron as calculated as FeO: at most 0.21%,        CoO: 3 to 35 ppm, and        Se: 1 to 15 ppm,        
wherein the luminous transmittance is at least 60% in 3.9 mm thickness, the dominant wavelength is from 480 to 559 nm, and the excitation purity is at most 8% in 3.9 mm thickness.
(2) A heat-absorbing glass plate (Patent Document 1) containing, as represented by mass % based on the following components:                SiO2: 66 to 75%,        Na2O: 10 to 20%,        CaO: 5 to 15%,        MgO: 0 to 5%,        Al2O3: 0 to 5%,        K2O: 0 to 5%,        total iron as calculated as Fe2O3: 0.17 to 0.65%,        divalent iron as calculated as FeO: at most 0.18%,        CoO: 15 to 55 ppm,        Se: 0 to 5 ppm, and        NiO: 50 to 350 ppm,        
wherein the luminous transmittance is at least 60% in 3.9 mm thickness, the dominant wavelength is from 480 to 580 nm, and the excitation purity is at most 8% in 3.9 mm thickness.
(3) A heat-absorbing glass plate (Patent Document 2) containing, as represented by mass % based on oxides:                SiO2: 66 to 75%,        Na2O: 10 to 20%,        CaO: 5 to 15%,        MgO: 0 to 5%,        Al2O3: 0 to 5%,        K2O: 0 to 5%,        total iron as calculated as Fe2O3: 0.40 to 1.0%,        CoO: 4 to 20 ppm, and        Cr2O3: 0 to 100 ppm,        
wherein the mass ratio (FeO/t-Fe2O3) of divalent iron as calculated as FeO to total iron as calculated as Fe2O3 is from 0.35 to 0.50, the luminous transmittance is at least 65% in 3.9 mm thickness, the dominant wavelength is from 485 to 489 nm, the excitation purity is from 3 to 18%, the total solar energy transmittance is at most 55% in 3.9 mm thickness, the total solar ultraviolet transmittance is at most 60% in 3.9 mm thickness, and the total solar infrared transmittance is at most 35% in 3.9 mm thickness.
(4) A heat-absorbing glass plate (Patent Document 3) containing, as represented by mass % based on the following components:                SiO2: 65 to 75%,        Na2O: 10 to 20%,        CaO: 5 to 15%,        MgO: 0 to 5%,        Al2O3: 0 to 5%,        K2O: 0 to 5%,        total iron as calculated as Fe2O3: 0.30 to 0.75%,        CoO: 0 to 15 ppm, and        Se: 1 to 15 ppm,        
wherein the mass ratio (FeO/t-Fe2O3) of divalent iron as calculated as FeO to total iron as calculated as Fe2O3 is from 0.26 to 0.675, the luminous transmittance is at least 65% in 3.9 mm thickness, the total solar energy transmission (TSET) is at most 65%, the standard transmitted color shift is less than 6, and the excitation purity is less than 8%.